Images acquired by magnetic resonance are seriously degraded by quasi-periodic motion in the subject. The best example of quasi-periodic motion is the breathing of a patient, however, there are other examples, such as but not limited to pulsitile blood flow. The scan sequence collected data that is effected by respiratory and other regular motions that are operated on by Fourier transforms produce image artifacts consisting of local blurring and more or less regularly spaced "ghost" images propagating along the direction of the phase encoding magnetic field gradient.
The prior art methods for reducing the artifacts caused by quasi-periodic motion include apparatus and methods of data acquisition which cause the acquired data to assume a quasi-linear function rather than the quasi-cyclical function. This is done, for example, by reordering the phase encoding gradient pulses. Instead of the usual sequential application of phase encoding pulses with different amplitudes, the gradient amplitudes are selected as a function of the physical position of the moving portion of the body being imaged. Such correction methods require additional transducers for sensing the position of the thorax of the patient for example and means for reordering the phase encoding gradients. In addition, "a learning period" is usually required in the reordering methods to determine the average displacement of the moving portions of the body. The learning period, of course, adds time to the scan sequence and lowers the throughput capability of the system.
An alternative method of correcting for the respiratory effects in two-dimensional Fourier transform MR imaging was described in an article by L. Axel et al entitled "Respiratory Effects in two Dimensional Fourier Transform MR Imaging" published in the Journal of Radiology, volume 160, pages pp795-801 (1986). The method described in that article uses averaging to overcome the adverse respiratory effects on the two-dimensional Fourier transformed image. More particularly, a pulse sequence including a phase encoding gradient having a first amplitude is used to obtain an echo signal. This step with the encoding gradient having the first amplitude is repeated a plurality of times. The output echo signals are then averaged to attempt to flatten the otherwise quasi-periodic modulation produced by the respiratory motion. Subsequently, the sequence is run a number of times with a second phase encoding gradient pulse amplitude. The echoes are again averaged. The scan sequences are run in this manner until an entire matrix of signal data is acquired.
The actual number of times in which the same phase encoding pulse amplitude is used is given by the quotient of TP/TR; where TP is the period of the quasi-cyclical motion and TR is the time to repeat of the scan sequence. For example, the respiration period is in the order of 4,000 milliseconds. A typical time to repeat is 500 milliseconds, therefore, the number of samples to be averaged will 8. In other words, each of the echo acquisitions is repeated 8 times in a typical example. There are circumstances when 6 times will accomplish the task of flattening the periodicity of the respiratory motion. Similarly on some subjects 16 samples will be required to flatten out the quasi-cyclical motion. Thus, the averaging system for reducing motion artifacts caused by quasi-periodic motion requires more acquisition time than a typical reordering system. Accordingly, when using the prior art averaging schemes transducers or gating equipment are not needed during the scans, they are needed to measure the total breathing movement. Also, the prior art averaging does increase the imaging time. Accordingly, scientists in the field are still searching for apparatus and methods for overcoming or at least more efficiently reducing the ghosting and blurring artifacts caused by quasi-periodic motion in the patient being imaged by magnetic resonance systems.